text-shadow in ::selection is supported by Chrome, Safari and Firefox 17+.

Though this pseudo-element was in drafts of CSS Selectors Level 3, it was removed during the Candidate Recommendation phase, as it appeared that its behavior was under-specified, especially with nested elements, and interoperability wasn't achieved (based on discussion in the W3C Style mailing list).

Examples

Gecko is the only engine requiring the prefix. Due to the fact that the CSS parsing rules require dropping the whole rule when encountering an invalid pseudo-element, two separate rules must be written: ::-moz-selection, ::selection {...}. The rule would be dropped on non-Gecko browsers as ::-moz-selection is invalid on them.

Specifications

The ::selection CSS pseudo-element was drafted for CSS Selectors Level 3 but removed before it reached the Recommendation status. Despite this, it's implemented in some browsers, which will probably retain experimental support for it.

Right now, the ::selection CSS pseudo-element is not in any specification on the standard track, however it has been approved to be added to Pseudo-elements Level 4 in the near future.